Add docs page source links

We had a ([somewhat broken](bluerobotics/ardusub-zola#139)) version of this with the previous docs, and it's a useful feature for encouraging people to find and contribute to the documentation (#36 is at least a partially helpful workaround in that direction in the meantime).

Since the new docs infrastructure doesn't submodule the content, the actual link creation part should be pretty straightforward, so we can presumably just copy [how Adidoks does it](https://github.com/search?q=repo%3AES-Alexander%2Fadidoks%20docs_edit_page&type=code).

We would need to decide what kind of button we want for it (e.g. "Edit this page"/"View docs source", possibly with a GitHub or edit pencil icon), and where that should be (e.g. navbar / sidebar / footer?). I'm leaning towards an edit pencil icon on the right side of the "On the page" title in the in-page navigation sidebar :-)

Start by comparing the Adidoks docs_edit_page implementation linked in the issue with this Zola theme's page layout and in-page navigation sidebar. Decide the link wording, icon, and placement, then verify that each documentation page exposes a working source or edit link.

Written by the indexing model from the issue text.